A development of the advanced information society increasingly makes secure information transmission means important. In order to ensure the security of information, a cipher technology such as a public key cryptography has been currently widely employed. The public key cryptography requires an astronomical calculation for decryption, which supports the security of information. However, when a computer is further developed in the future, the decryption that cannot be currently performed may be enabled.
One of methods for coping with the above problem is to use a communication manner based on a quantum-mechanical principle. There have been various optical communication methods using the quantum mechanical manner.
From the viewpoint of the detecting method, the quantum optical communication is classified into two systems: one system requires a reference light and another system requires no reference light. In the system that requires the reference light, it is necessary that the signal light and the reference light are equal in the wavelength to each other, and the phases are synchronous with each other. The methods of obtaining the above reference light have been variously proposed and developed at a stage of developing a coherent optical communication system (Document “Sadakuni Shimada, Coherent optical communication, pp 49-50, published by Corona Corp. in 1988”).
The basic structures for that system are that a local light source for the reference light is located at a detecting section, and the frequencies and the phases of a signal light and a local light source are adjusted within a given range by using a sophisticated electric circuit, which is a very difficult method. In order to minimize the difficulty, there is a method using a part of the signal light when generating the reference light (Document “Sadakuni Shimada, Coherent optical communication, pp 25-26, published by Corona Corp. in 1988”). However, when a part of the signal light is used for generation of the reference light, because the signal is quantum-mechanically destroyed depending on the used amount of signal light, the method using a part of signal light for generation of the reference light cannot be basically applied to the quantum information.
In general, as the reference light, a part of an output light from a light source that is used to generate the signal light is used, in the experiment of the quantum communication which is performed in a laboratory. With this structure, the signal light and the reference light are perfectly synchronous with each other, and the conditions needed for the reference light are satisfied. However, in the case where a method of transmitting the signal light and the reference light in different optical paths is used in a long-haul transmission out of the laboratory, the synchronization of the phases are not guaranteed because of the fluctuation of the phases which are attributable to a difference in the external environments of the respective optical paths. This problem is one of reasons that the quantum communication system that requires the reference light cannot be developed to a field experiment that is conducted out of a laboratory and further to a practical application stage.
In order to solve the above problem, there has been proposed a method in which the signal light and the reference light are generated with the same light source as a seed light, and those lights are transmitted in the same transmission path with a time lag (Document “T. Hirano, H. Yamanaka, M. Ashikaga, T. Konishi, and R. Namiki: Quantum cryptography using pulsed homodyne detection, Physical Review A 68, 042331 (2003)). As a result, the external environmental factors in the transmission are equal to each other, and the synchronization of the signal light and the reference light in the phase after the long-haul transmission is improved.
However, even in this method, the synchronization of the signal light and the reference light are not perfect. The nonlinearity of the optical fiber is generally small but becomes large as the net for the long-haul transmission due to the integral effect. The signal light and the reference light are different in the intensity, and moreover, the phase characteristic after transmission is different between the respective parts of reference light due to the intensity fluctuations through the nonlinearity effect.
In addition, there generally arises such a problem on the loss in the quantum communication in addition to a problem on the phase synchronization. When the above quantum signal is partially extracted, the quantum state is destroyed as much as the extracted amount. This supports that the quantum communication is secure. Even if the quantum signal is not intentionally partially extracted, the signal is partially destroyed by the transmission loss. Accordingly, in order to transmit the quantum signal at a long distance, a breakthrough technique is required.